There is a general need of superhard materials for many different fields of application. The materials may be working on or being in contact with steel, non-iron metals, paper, polymers, concrete, stone, marble, soil, cemented carbide and grinding wheels of aluminum oxide, silicon carbide, diamond or cubic boron nitride etc.
Commonly synthetic diamond powders are produced having sizes less than 600 .mu.m. For several fields of application, for instance drilling, grinding, machining of leather, rubber, and wood, larger grains are needed. Grains comprising diamond can be produced from diamond composite materials comprising a diamond skeleton (self-bonded diamonds with a small amount of or no bonding material) or comprising diamond particles bonded by a matrix comprising ceramic phases produced by sintering diamond particles in the presence of such materials, by high pressure and high temperature methods, or by hot press methods.
Large abrasive grains are produced from diamond composite materials like Ballas, Carbonado etc. with fine diamond particles. These materials are e.g. produced by sintering in high-pressure chambers, with subsequent crushing of the composite and classification of the abrasive grains.
Because of the diamond instability and tendency to graphitize, the heat treatment is done in conditions of diamond stability at high temperatures, 1300-1600.degree. C., in high-pressure chambers with pressures of 30.000-60.000 atm (HP/HT).
The drawbacks of methods using high pressure is that the manufacturing technology is rather complex and requires special equipment, e.g. presses and dies. The consequenses are high production costs, limited production capacity, and limited shapes and sizes of the diamond composite bodies.
There are some patents describing the production of abrasive grains:
A method for producing diamond-containing abrasive grains is disclosed in the patent EPO 0,435,501. The method includes crushing of a diamond compact consisting of a diamond skeleton being 70-90% by volume and silicon, silicon carbide and/or metal silicide preferably to small sized fragments of about 1,5 mm. The diamond compact is preferably made by sintering a mixture of diamond, silicon, silicon carbide and/or metal silicide in a high-pressure chamber. After crushing of the diamond compact the fragments may be sintered as they are in HP/HT conditions to give a hard product. Alternatively a metal or second phase may be included to the fragments to infiltrate the fragments during the sintering and compacting. The abrasive grains produced by this method may have insufficient strength due to a high content of diamond in the crushed compact. The initial diamond in the compact has been sintered to form a continuous skeleton. A diamond skeleton is brittle and therefore the grains may be brittle. Furthermore, the compact is made with a high-pressure/high-temperature process.
U.S. Pat. No. 4,224,380 describes the production of a compact of self-bonded abrasive particles (diamonds and/or CBN) with an interconnected network of pores dispersed throughout. The compact is produced by bonding a mass of abrasive particles into a self-bonded body through the use of a sintering aid material under high pressure and high temperature (HP/HT). The body includes said particles in a self-bonded form and said material infiltrated throughout the body. The body is then treated to remove the infiltrated material to thereby produce a compact consisting of the self-bonded abrasive particles. The drawback of this method is the use of high pressure and high temperature.
Several patents reveal techniques to produce diamond abrasive grains without using high pressure and high temperature:
U.S. Pat. No. 3,520,667 describes the production of silicon carbide coated diamond abrasive grains by suspending the diamond particles in a gaseous atmosphere comprising a volatile silicon compound and forming by thermal decomposition of the silicon compound a silicon carbide layer on the particles. The decomposition takes place preferably by forming a fluidised bed with diamonds suspended in a mixture of gases, which includes hydrogen and the volatilised silicon compound. The bed is heated to 1300-1500.degree. C. to cause decomposition of the silicon compound and the formation of the silicon carbide coating on the dispersed and suspended diamond particles. Drawbacks with this method of producing grains is that the silicon carbide coated diamonds are individual particles and not bonded to each other into agglomerates, thereby reducing the size of the abrasive grit and the subsequent application field.
U.S. Pat. No. 4,606,738 and EPO 0,061,605 describes composite abrasive particles comprising a core abrasive crystal (diamonds or CBN) and a silicon carbide coating on said core crystal. The abrasive particles are preferably made by infiltrating core crystals coated with non-diamond carbonaceous material with fluid silicon. Then the silicon is leached out from the produced mass of core crystals and matrix of silicon carbide and silicon. The resulting leached mass is sub-divided and the composite abrasive particles are recovered. Another disclosed embodiment is the aggregates of the composite abrasive particles that are interconnected by a matrix of silicon carbide, which has an open structure. Drawbacks of this method are that the produced abrasive grains, i.e. diamond particles coated by silicon carbide, have sizes equivalent to the size of the initial diamond particles. Thus there is a limit for preparing large, cheap abrasive grains of sizes several times greater than that of initial diamond particles and greater than industrially produced diamond particles. Aggregates produced by the given method are porous and have not high strength, which limit their field of application.